Tuesday, June 13, 2006

Polar Bear Cannibalism

Many years ago, a student of my advisor's did a study on mice hiding in cracks in the ground that form in the hot Kansas summer. It was a nice little study that he could do while he had threads attached to the mice to track them. He wrote it up, and sent it off. LIke a lot of papers, it got rejected. He submitted it to some other places, but it didn't go anywhere.

Then he wrote it up as an article about the potential effects of climate change, and it got published in a major journal of conservation biology.

I don't know what that says about the world. It was good research and deserved to get published.

I was reminded of this by media coverage of a a study by a group of arctic scientists that turned up three instances of polar bears killing and eating one another. The scientists report that "Given the absence of observations of polar bears stalking and killing other polar bears during all prior years of polar bear study in the Beaufort Sea, it seemed unusual that we observed three such instances within a two and a half month period in 2004."

They suggest that this may be a result of climate change. Less available food has been found to cause cannibalism in polar bears and other bears before. Melting sea ice reduces the area that bears, especially the young males implicated in these attacks, can use for hunting.

There are plenty of previous cases of cannibalism in polar bears, though not in the study area. This is research deserving of publication, but its major novelty is geographic, not behavioral. The other interesting scientific result is that this is the first report of a polar bear killing a female in the den she gave birth in.

That result turns out not to be terribly surprising though.

A 1999 study from Norway found three cubs killed in their den. They had nursed recently, which indicates that the mother had not abandoned them, but there was no evidence of a fight between the mother and the attacker. The authors of that study also reported a videotape of a large male attacking a young female and her yearling offspring. He chased both, probably fighting with the female, then killed and ate the cub. They also point out that in 1988, Stirling reported a female killed defending her cubs from attack.

A 2002 report of polar bear cannibalism in Canada gives a similar picture. An adult male with wounds on the head, neck and shoulder was observed feeding on a yearling cub, and a female was seen nearby with wounds on her forelimbs. Again, it appears that the male attacked the female in order to drive her away from a cub and eat it. While this is the first fatality from such an attack, it's not an entirely surprising outcome.Unfortunately, the finding is being presented in a way that is far too extreme. A Yahoo report says that "Deborah Williams of Alaska Conservation Solutions, a group aimed at pursuing solutions for climate change, said the study represents the 'bloody fingerprints' of global warming."

This is equivalent to saying that the victims of Katrina represent the bloody fingerprints of global warming. It's accurate only in the broadest statistical sense. Reports of polar bear cannibalism in the literature go back to the 1970s, and careful observation that would detect it probably doesn't date too far before that. It may be that it will become more common as a result of global warming, just as hurricanes will become more intense as a result, but attributing individual deaths to a long-term statistical process is too vague for phrases like "bloody fingerprints."